Open-File Report 98-791: Physical Characteristics of Dungeness Crab and Halibut Habitats in Glacier Bay, Alaska

Analysis of Side Scan Imagery (An Example)

In a future report, we will characterize the benthic habitat using a combination of the side-scan data, the profiling data, onshore geologic mapping by Brew et al. (1978), and previous marine geology work (e.g. Carlson et al., 1977; Cai et al., 1997). Final interpretations will be in the form of georeferenced habitat polygons which will be combined with the existing Geographic Information System (GIS) database (Geiselman et al., 1997). Our interpretation of the geophysical data will be based on the experience garnered in studying similar acoustic data from Glacier Bay and the Gulf of Alaska (e.g. Carlson et al., 1992, Carlson et al., 1980). We are using a variety of techniques (scuba, delta submersible, rebreathers, drop camera) to visually confirm our interpretation of the geophysical data. We plan to test the predictive value of the interpretation by conducting fish population studies in the areas where the geophysical data interpretations exist.

In this report we present an analysis of the sidescan image collected to the west of Strawberry Island (see above). Based on geologic mapping on Strawberry Island, we know that the surficial sediments are Quaternary age (Brew et al., 1978). Possible interpretations of the sidescan image include layered or structurally deformed bedrock, lateral moraines, or sediments grooved by the passage of large icebergs. A bedrock seafloor habitat will support a much different benthos than that supported by other adjacent bottom types including lateral moraines, or sediments grooved by the passage of large icebergs. Examination of the sub-seafloor seismic data (150 Kbytes) shows a prominent sub-seafloor reflection which rules out exposed bedrock as a possible interpretation, except at two locations where the reflector may intersect the sea floor at distances of 7 m and 68 m. Samples of the rock are needed to define what geologic units underlie the onshore sediment. Sampling, planned next year, with closed-circuit diving equipment (i.e. rebreathers) may provide bedrock samples from the area.

The criss-crossing pattern of grooves seen in the sidescan image east of Strawberry Island rules out lateral moraines as an interpretation of the seafloor habitat. Our preferred interpretation of the data is that the seafloor is composed of coarse sediment grooved by the passage of large icebergs. The earliest period when large icebergs would have calved up stream of this area is when the glacier terminus was in the Strawberry Island area between 1794 and 1845 (American Geographical Society, 1966). Sediment grooving could have occurred in more recent times as icebergs calving further up stream passed Strawberry Island on their way south. Using the habitat characterization scheme of Greene et al. (1995), this habitat would be described as Intermediate shelf, grooved gravel and boulder, flat bottom, with probable winnowing by tidal and riparian currents.

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For more information, contact the PCMSC team.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Suggested citation:
Cochrane, Guy R., Carlson, Paul R., Denny, Jane F., Boyle, Michael E., Taggart, James S., and Hooge, Philip N., 1998, Physical Characteristics of Dungeness Crab and Halibut Habitats in Glacier Bay, Alaska: U.S. Geological Survey Open-File Report 98-791, https://pubs.usgs.gov/of/1998/0791/.

U.S. Department of the Interior
SALLY JEWELL, Secretary

U.S. Geological Survey
Suzette M. Kimball, Acting Director